40 ILCS 5/8-176

The board shall also ascertain the service rendered by each present employee who did not attain age 65 prior to July 1, 1923, and the service rendered by each future entrant between the day before the effective date and July 1, 1923. Each such employee shall be credited with 5 3/4% of his annual salary during such service, with interest thereon at the effective rate to July 1, 1923, upon the assumption that 1/12 of such 5 3/4% of such annual salary was due at the end of each month of such service.